Wireless communication system, mobile station, base station, and wireless communication method

ABSTRACT

A mobile station including: a communication unit that includes a first logical processing subject and a second logical processing subject that operates in association with the first logical processing subject and performs multiple communications with a first wireless communication apparatus and a second wireless communication apparatus; an accumulation unit that accumulates data received from the second wireless communication apparatus; and a notifying unit that notifies the first wireless communication apparatus of data receiving status in the accumulation unit.

This application is continuation application of U.S. Utility patentapplication Ser. No. 16/210,383, filed on Dec. 5, 2018, which iscontinuation application of U.S. Utility patent application Ser. No.14/933,582, filed on Nov. 5, 2015, now U.S. patent Ser. No. 10/212,749issued on Feb. 19, 2019, which is a continuation of InternationalApplication PCT/JP2013/063218 filed on May 10, 2013 and designating theU.S., the entire contents of each are incorporated herein by reference.

FIELD

The present invention relates to a wireless communication system, amobile station, a base station, and a wireless communication method.

BACKGROUND

Conventionally, various efforts have been made to increase thetransmission capacity of a wireless communication system (hereinafter,may be referred to as a “system capacity”). For example, in the 3rdGeneration Partnership Project Radio Access Network Long Term Evolution(3GPP LTE), discussion has been made on a technique for increasing thesystem capacity by utilizing a “small cell (SC)” in addition to a“macrocell”. A “cell” herein is a term representing a range covered by awireless base station for transmission and reception of wireless signalsby wireless terminals. The wireless base station and the cell indicatesubstantially corresponding concepts; therefore, in the explanationbelow, the “cell” may be interpreted as the “wireless base station” asappropriate. Further, the “macrocell” is a base station that can performtransmission with high transmission power, that is, a cell of a basestation with a large coverage area. Furthermore, the “small cell” is abase station that performs transmission with low transmission power,that is, a cell of a base station with a small coverage area.

In the 3GPP LTE, as a configuration of a mobile communication system, aconfiguration including a plurality of small cells in a macrocell hasbeen studied, for example. Further, a technique for simultaneouslyconnecting a mobile station to a macrocell and a small cell has beenstudied. Furthermore, a technique for simultaneously connecting a mobilestation to two different small cells has been studied. In this manner,communication performed by a mobile station simultaneously connecting totwo different cells may be referred to as a dual connection (dualconnectivity) in some cases. Alternatively, as the dual connectivity,there is a case where a base station directly connects to a higher-layerdevice. However, the configuration is not limited to the above, and, ingeneral, the dual connectivity means that a terminal connects to andsimultaneously communicates with a plurality of base stations tosimultaneously transmit and receive different kinds of information toand from the base stations. In the explanation below, the dualconnectivity is described; however, the same discussion can be appliedto multiple connectivity such as triple or more connectivity. Therefore,the dual connectivity in the description below may be regarded as aconcept including multiple connectivity, or the dual connectivity may beinterpreted as multiple connectivity.

If a mobile station simultaneously connects to a macrocell and a smallcell, for example, a control plane, which is a call to transmit L3control information to set a transmission channel for transmitting dataor to control a handover, is connected to the macrocell. Further, a dataplane, which is a call to transmit and receive data, is connected to thesmall cell. Meanwhile, the control plane may be referred to as a C-planeor a signaling radio bearer (SRB). Further, the data plane may bereferred to as a user plane, a U-plane, or a data radio bearer (DRB).

Furthermore, if a mobile station simultaneously connects to twodifferent small cells, for example, it may be possible to employ aconfiguration in which the control plane is connected to one of thesmall cells and the data plane is connected to the other one of thesmall cells, or a configuration in which the data plane is connected toboth of the base stations. As described above, in general, the dualconnectivity means that a terminal connects to and simultaneouslycommunicates with a plurality of base stations to simultaneouslytransmit and receive different kinds of information to and from the basestations.

In this manner, in the dual connectivity, a wireless base station towhich a control plane is connected may be referred to as a primarywireless base station (cell). Further, a wireless base station (cell) towhich a data plane for performing data communication in cooperation withthe primary wireless base station (cell) may be referred to as asecondary wireless base station. Furthermore, these base stations may berespectively referred to as an anchor wireless base station (cell) andan assisting wireless base station (cell). Moreover, these base stationsmay be respectively referred to as a master wireless base station (cell)and a slave wireless base station (cell). Incidentally, in the case ofthe dual connectivity, to improve the communication characteristics, ifa wireless communication terminal has a capability to receive pieces ofdata from two wireless base stations, it may be possible to employ aconfiguration in which each of the primary wireless base station and thesecondary wireless base station outputs data to the wirelesscommunication terminal. However, names of the wireless base stations arenot limited to those described above. In general, as in a conventionalLTE communication system, if a wireless base station that connects toand performs communication with both of a control plane and a data planeserves as a main base station, various names may be employed withoutdeparting from this intention. Hereinafter, for the sake of simplicity,terms “primary” and “secondary” are used.

As the configuration of the dual connectivity, various configurationshave been proposed depending on at which of layers the data plane isseparated.

For example, there is a configuration in which the data plane isseparated in a stage before a packet data convergence protocol (PDCP)layer. Further, for example, there is a configuration in which the dataplane is separated between the PDCP layer and a radio link control (RLC)layer. Furthermore, for example, there is a configuration in which thedata plane is separated between the RLC layer and a medium accesscontrol (MAC) layer. The configurations are not limited to the above,and it is possible to employ a configuration in which separation isperformed within a layer. For example, it may be possible to employ aconfiguration in which a part of the functions of the PDCP layer isimplemented by a primary base station, and the rest of the functions ofthe PDCP layer is implemented by a secondary base station. The sameapplies to the RLC layer and the MAC layer.

If the configuration of the dual connectivity is employed, a primarywireless base station and a secondary wireless base station areconnected by a wired link or a wireless link. Then, after the data planeis separated, data is sent to the secondary wireless base station viathe link connecting the primary wireless base station and the secondarywireless base station.

Non Patent Document 1: 3GPP TR 36.932 V12.1.0 (2013-03), 3rd GenerationPartnership Project; Technical Specification Group Radio Access Network;Scenarios and requirements for small cell enhancements for E-UTRA andE-UTRAN (Release 12)

However, in the dual connectivity, in the secondary wireless basestation, the MAC layer may acquire, from the RLC layer, pieces of datacorresponding to a transmittable amount and transmit the pieces of dataon the basis of an amount of transmitted data notified by a physical(PHY) layer. Alternatively, the MAC layer may acquire, from the RLClayer, pieces of data corresponding to a transmittable amount andtransmit the pieces of data on the basis of transmittable datacorresponding to a wireless quality notified by the PHY layer. In thiscase, the PDCP layer acquires a retention amount of data in the RLClayer, and thereafter sends pieces of data corresponding to the acquiredretention amount to the RLC layer. In this configuration, the PDCP layeracquires the retention amount of data in the RLC layer via the linkconnecting the primary wireless base station and the secondary wirelessbase station. However, when the communication quality of the linkconnecting the primary wireless base station and the secondary wirelessbase station is not high, acquisition of the retention amount of data inthe RLC layer by the PDCP layer may be delayed. In this case, the amountof data transmitted from the PDCP layer is not controlled appropriately,and data delivery from the PDCP layer may be delayed. Further, it isdifficult to reflect the latest state of the retention amount of data inthe RLC layer, so that a greater amount of data than the amount that canbe stored in the RLC layer (buffering is possible) may be delivered fromthe PDCP layer to the RLC layer and the data may be discarded.

SUMMARY

According to an aspect of an embodiment, a wireless communication systemincludes: a first wireless communication device; and a second wirelesscommunication device, wherein the first wireless communication deviceincludes: a first communication unit that includes a first logicalprocessing subject and a second logical processing subject that operatesin association with the first logical processing subject, and performsmultiple communications with the second wireless communication device;and a notifying unit that notifies the second wireless communicationdevice of data related information, and the second wirelesscommunication device includes: a second communication unit thattransmits data; and a control unit that receives the data relatedinformation from the notifying unit and controls an amount of datatransmitted by the second communication unit.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a wireless communication system accordingto a first embodiment.

FIG. 2 is a conceptual diagram collectively illustrating variousconfigurations of the wireless communication system.

FIG. 3 is a schematic diagram illustrating dual connectivity in awireless communication system according to a second embodiment.

FIG. 4 is a block diagram of the wireless communication system accordingto the second embodiment.

FIG. 5 is a diagram illustrating transmission and reception of user dataperformed by using each of link layers in the wireless communicationsystem according to the first embodiment.

FIG. 6 is a diagram for explaining the flow of pieces of user data towhich numbers are regularly assigned.

FIG. 7A is a diagram illustrating an example of a PDCP status report fora 12-bit sequence number.

FIG. 7B is a diagram illustrating an example of a PDCP status report fora 15-bit sequence number.

FIG. 7C is a diagram illustrating an example of a PDCP status report fora 7-bit sequence number.

FIG. 8 is a diagram illustrating information stored in a PDU Type.

FIG. 9 is a sequence diagram for explaining the overall flow to controla delivery amount of data.

FIG. 10 is a flowchart to control an amount of user data delivered to asmall base station in the communication system according to the secondembodiment.

FIG. 11A is a diagram for explaining transmission of user data at thetime of dual connectivity in a conventional communication system.

FIG. 11B is a diagram for explaining transmission of user data at thetime of dual connectivity in the communication system according to thesecond embodiment.

FIG. 12 is a hardware configuration diagram of a base station.

FIG. 13 is a hardware configuration diagram of a mobile station.

FIG. 14 is a schematic diagram illustrating dual connectivity in awireless communication system according to a modification of the secondembodiment.

FIG. 15 is a diagram illustrating transmission and reception of userdata performed by using each of link layers in a wireless communicationsystem according to a third embodiment.

FIG. 16 is a diagram illustrating transmission and reception of userdata performed by using each of link layers in a wireless communicationsystem according to a fourth embodiment.

FIG. 17 is a diagram illustrating transmission and reception of userdata performed by using each of link layers in a wireless communicationsystem according to a fifth embodiment.

FIG. 18 is a diagram illustrating an example of a format of atransmission packet.

FIG. 19 is a diagram illustrating a format of an RLC status report.

FIG. 20 is a diagram illustrating contents stored in CPT.

FIG. 21 is a diagram for explaining an example of a method ofcalculating a retention amount of data.

FIG. 22 is a sequence diagram for explaining the overall flow totransmit the RLC status report in the wireless communication deviceaccording to the fifth embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of a wireless communication system, a mobilestation, a base station, and a wireless communication method disclosedin the present application will be described in detail based on thedrawings. Incidentally, the wireless communication system, the mobilestation, the base station, and the wireless communication methoddisclosed in the present application are not limited by the embodimentsbelow.

First Embodiment

First, a method of realizing dual connectivity will be brieflydescribed. Carrier aggregation (CA) that is a technique defined in aconventional LTE system will be discussed. The carrier aggregation is torealize high-speed/large-capacity communication by using aggregatedcomponent carriers (CC) or cells that are frequency bands used forcommunication between a wireless base station and a wireless terminal.The bandwidth supported by the LTE system is limited to 20 MHz at themaximum; however, due to introduction of the carrier aggregation, itbecomes possible to use a bandwidth of 40 MHz by aggregating two CCs of20 MHz, for example.

In the framework of the carrier aggregation, it seems possible torealize the dual connectivity by, for example, causing a macrocell touse one CC and causing a small cell to use another CC. However, it isconsidered difficult to realize the dual connectivity based on thecarrier aggregation for reasons as described below.

Here, the carrier aggregation is considered from the perspective of aprotocol stack in an LTE system. The protocol stack in the LTE systemincludes, in order from the lowest layer, a PHY layer, a MAC layer, anRLC layer, and a PDCP layer (higher-level layers are also provided, butare omitted herein). As correspondence with a commonly-used open-systemsinterconnection (OSI) reference model, the PHY layer in the LTE systemcorresponds to a physical layer that is the first layer of the OSIreference model. Further, the MAC layer, the RLC layer, and the PDCPlayer in the LTE system correspond to a data link layer that is thesecond layer of the OSI reference model. The MAC layer is responsiblefor a scheduler function or the like, the RLC layer is responsible forsequence control or the like, and the PDCP layer is responsible forsecurity or the like.

If the carrier aggregation is viewed from the perspective of theprotocol stack, transmission data is split in the physical layer.Further, reception data is integrated in the physical layer. This meansthat, in the carrier aggregation, there are a plurality of entities inthe physical layer and there is a single entity in a higher-level layer,such as the MAC layer, in both of a transmission side and a receptionside. Here, the entity is a term that means a logical (or virtual)processing subject. The entity exists in each of the layers of theprotocol stack, and does not always have a one-to-one correspondencewith an apparatus that is a physical processing subject, but may have anN-to-one correspondence. For example, according to the carrieraggregation as described above, there are a plurality of entities in thephysical layer in both of a transmission side and a reception side.

Incidentally, in the protocol stack for general data communication inthe LTE system, entities in the PHY layer, the MAC layer, the RLC layer,and the PDCP layer operate in one serial line in each of the wirelessbase station and the wireless terminal.

In contrast, in data communication based on the carrier aggregation inthe LTE system, entities in the PHY layer, the MAC layer, the RLC layer,and the PDCP layer operate in each of the wireless base station and thewireless terminal. However, the data communication based on the carrieraggregation differs from the general data communication in the LTEsystem in that only the physical layer is split into two entities. Inthis manner, if the carrier aggregation is viewed from the perspectiveof the protocol stack, transmission data is split in the PHY layer andreception data is integrated in the PHY layer.

Meanwhile, as described above, the MAC layer in the LTE system isresponsible for the scheduler function. The scheduler function is afunction to determine what data is transmitted at which timing and atwhich frequency. As described above, a single entity exists in the MAClayer in the carrier aggregation, and this means that there is a singlescheduler.

If the dual connectivity is to be realized by the carrier aggregation,for example, a MAC entity (scheduler) that exists in a macro wirelessbase station performs scheduling on PHY entities (CCs) that exist ineach of the macro wireless base station and a small wireless basestation. This can hardly be realized because of a latency issue incommunication between wireless base stations. This is because thescheduling in the LTE system needs to be performed with an extremelyshort period in units of 1 millisecond (1 subframe). Therefore, in thecarrier aggregation, it is possible to cause a single wireless basestation to perform transmission and reception by using a plurality ofcarriers, but it is not practical to cause a plurality of wireless basestations to perform transmission and reception by using a plurality ofcarriers. In view of the above, it may be extremely difficult to realizethe dual connectivity based on the carrier aggregation.

Incidentally, based on the consideration on the carrier aggregation asdescribed above, to realize the dual connectivity, data is split in adata link layer above the physical layer. As described above, in the LTEsystem, the data link layer is divided into three layers of the MAClayer, the RLC layer, and the PDCP layer. For example, if data is splitin the MAC layer, a plurality of entities exist in the MAC layer.Accordingly, a plurality of schedulers are provided, so that it becomespossible to provide different schedulers to the macro wireless basestation and the small wireless base station, for example. Therefore, bysplitting data in the MAC layer, it becomes possible to avoid theabove-described issue based on the latency in communication betweenwireless base stations, and the dual connectivity can be realized.Similarly, it is possible to realize the dual connectivity when data issplit in the RLC layer or the PDCP layer.

Incidentally, data splitting in the data link layer is not equivalent tothe dual connectivity. This is because, as in the case where a singlewireless base station has a plurality of MAC entities, singleconnectivity may be realized even when data is split in the data linklayer.

Next, with reference to FIG. 1 , a wireless communication systemaccording to a first embodiment will be described. FIG. 1 is a blockdiagram of the wireless communication system according to the firstembodiment. As illustrated in FIG. 1 , the wireless communication systemaccording to the embodiment includes a wireless communication device 1,a wireless communication device 2, a wireless communication device 3,and a higher-layer communication device 4.

The wireless communication device 1 and the wireless communicationdevice 2 implement dual connectivity. The wireless communication device1 is a primary wireless communication device in the dual connectivity,and the wireless communication device 2 is a secondary wirelesscommunication device in the dual connectivity.

The wireless communication device 1 and the wireless communicationdevice 2 are connected by wire, for example. The wireless communicationdevice 1 and the wireless communication device 2 transmit and receivedata to and from each other by using a wired link connecting betweenthem. Further, the wireless communication device 1, the wirelesscommunication device 2, and the wireless communication device 3 arewirelessly connected.

The wireless communication device 1 includes a communication unit 11including a receiving unit 12 and a transmitting unit 13, and includes acontrol unit 14.

The receiving unit 12 receives control data and user data from thehigher-layer communication device 4. The receiving unit 12 outputs thereceived control data and the received user data to the transmittingunit 13. The control data may be data generated by the wirelesscommunication device 1.

Further, the receiving unit 12 receives data related information (forexample, information related to data, such as information indicating aretention amount of data in a buffer or a data reception state) held bya receiving unit 32 of the wireless communication device 3, from atransmitting unit 33 of the wireless communication device 3. Then, thereceiving unit 12 outputs the data related information received from thetransmitting unit 33 to the control unit 14.

The transmitting unit 13 transmits the control data to the receivingunit 32 of the wireless communication device 3. Further, thetransmitting unit 13 receives, from the control unit 14, a notificationof an amount of data to be delivered to the wireless communicationdevice 2. Then, the transmitting unit 13 transmits data corresponding tothe amount of data to be delivered, which is notified by the controlunit 14, to the wireless communication device 2 via the wired link.

The control unit 14 integrally controls operations of the communicationunit 11 including the receiving unit 12 and the transmitting unit 13.Further, the control unit 14 receives the data related information heldby the receiving unit 32 of the wireless communication device 3 from thereceiving unit 12. Then, the control unit 14 determines an amount ofdata to be delivered to the wireless communication device 2 by using thedata related information received from the receiving unit 12.Thereafter, the control unit 14 notifies the transmitting unit 13 of thedetermined amount of data to be delivered to the wireless communicationdevice 2.

The wireless communication device 2 includes a communication unit 21including a receiving unit 22 and a transmitting unit 23, and includes acontrol unit 24.

The receiving unit 22 receives data from the transmitting unit 13 of thewireless communication device 1 via the wired link. Then, the receivingunit 22 outputs the received data to the transmitting unit 23.

The transmitting unit 23 receives input of data from the receiving unit22. Then, the transmitting unit 23 transmits the received data to thereceiving unit 32 of the wireless communication device 3.

The control unit 24 integrally controls operations of the communicationunit 21 including the receiving unit 22 and the transmitting unit 23.

The wireless communication device 3 includes a communication unit 31including the receiving unit 32 and the transmitting unit 33, andincludes a control unit 34.

The receiving unit 32 receives data from the transmitting unit 23 of thewireless communication device 2. Then, the receiving unit 32 transmitsthe data related information held therein (for example, informationrelated to data, such as information indicating a retention amount ofdata in a buffer or a data reception state) to the control unit 34.

The transmitting unit 33 receives the data related information held bythe receiving unit 32 from the control unit 34. Then, the transmittingunit 33 transmits the data related information held by the receivingunit 32 to the receiving unit 12 of the wireless communication device 1.

The control unit 34 integrally controls operations of the communicationunit 31 including the receiving unit 32 and the transmitting unit 33.Further, the control unit 34 acquires the data related information heldby the receiving unit 32. Then, the control unit 34 transmits the datarelated information held by the receiving unit 32 to the transmittingunit 33.

FIG. 2 is a conceptual diagram collectively illustrating variousconfigurations of the wireless communication system. Wirelesscommunication devices 1-1 to 1-N in FIG. 2 correspond to the wirelesscommunication devices 1 to 3 in FIG. 1 . As illustrated in FIG. 2 , itis possible to separate the flow of data at each entity or any pointbefore and after the entity to realize the dual connectivity, and ineach case, the wireless communication system according to the embodimentcan be applied appropriately. Further, as illustrated in FIG. 2 , thewireless communication system according to the embodiment can be appliedregardless of uplink communication or downlink communication.

As described above, in the wireless communication system according tothe embodiment, the wireless communication device 3, such as a mobilestation, notifies the wireless communication device 1 of information ona buffer (for example, information related to data, such as informationindicating a retention amount of data in the buffer or a data receptionstate), and the wireless communication device 1 controls an amount ofdata delivered to the wireless communication device 2, in accordancewith the notified information. The wireless communication device 3 andthe wireless communication device 1 wirelessly communicate with eachother; therefore, the wireless communication system according to theembodiment can send a notice to the wireless communication device 1 thatcontrols the delivery amount at high speed, as compared to a case wherethe wireless communication device 2 notifies the wireless communicationdevice 1 of information (for example, information related to data, suchas information indicating a retention amount of data in a buffer or adata reception state) that can be received via the wired link.Therefore, in the wireless communication system according to theembodiment, the wireless communication device 1 can send data with amore appropriate delivery amount to the wireless communication device 2,as compared to a case where data is transmitted to the wirelesscommunication device 2 by using information received via the wired link.That is, the wireless communication system according to the embodimentcan improve efficiency of communication between wireless communicationdevices.

Second Embodiment

Next, a second embodiment will be described. FIG. 3 is a schematicdiagram illustrating dual connectivity in a wireless communicationsystem according to a second embodiment. The wireless communicationsystem according to the embodiment includes a macro base station 100 asthe wireless communication device 1 in FIG. 1 . Further, a small basestation 200 is provided as the wireless communication device 2 in FIG. 1. Furthermore, a mobile station 300 is provided as the wirelesscommunication device 3 in FIG. 1 .

The mobile station 300 connects to the macro base station 100 as aprimary base station. The mobile station 300 is connected to the macrobase station 100 by a control plane 350 indicated by a solid arrow and auser plane 351 indicated by a dashed arrow. Further, the mobile station300 connects to the small base station 200 as a secondary base station.The mobile station 300 is connected to the small base station 200 by theuser plane 351. The user plane 351 in FIG. 3 indicates each of the userplanes 351 that connect the mobile station 300 and the small basestation 200 when a handover is performed. The control plane 350corresponds to an example of a “first logical processing subject”.Further, the user plane 351 corresponds to an example of a “secondlogical processing subject”.

The configuration of the wireless communication system as illustrated inFIG. 3 is often employed to reduce the number of times of trafficoffload or handovers.

Next, with reference to FIG. 4 , the wireless communication systemaccording to the embodiment will be described in detail. FIG. 4 is ablock diagram of the wireless communication system according to thesecond embodiment. In the wireless communication system according to theembodiment, in FIG. 4 , the same components denoted by the same symbolsas those in FIG. 1 have the same functions unless otherwise specified.

A macrocell that is a cell of the macro base station 100 includes aplurality of small cells including a cell of the small base station 200.The wireless communication system according to the embodiment implementsdual connectivity with the mobile station 300 by employing the macrobase station 100 as a primary wireless communication device and thesmall base station 200 as a secondary wireless communication device.

Further, the macro base station 100, the small base station 200, and themobile station 300 perform communication by using a link layer protocolcorresponding to a plurality of link layers, such as the PDCP layer, theRLC layer, the MAC layer, and the PHY layer.

Herein, with reference to FIG. 5 , a process performed for each linklayer will be described below. FIG. 5 is a diagram illustratingtransmission and reception of user data performed by using each of thelink layers in the wireless communication system according to the secondembodiment. Incidentally, in the embodiment, a case will be described inwhich the mobile station 300 acquires user data from each of the macrobase station 100 and the small base station 200.

The receiving unit 12 and the transmitting unit 13 in the communicationunit 11 of the macro base station 100 transmit and receive data by usingthe PDCP layer, a downlink RLC layer, the MAC layer, and the PHY layer.In FIG. 5 , for convenience of explanation, a PDCP layer 101, an RLClayer 102, an RLC layer 103, and a MAC layer 104 among layers forperforming communication by the communication unit 11 are illustrated.The RLC layer 102 is a downlink RLC layer, and the RLC layer 103 is anuplink RLC layer.

Incidentally, this configuration is a configuration in which an RLCacknowledged mode (AM) is used in the RLC layers. In the RLC AM, aninteractive bearer (bi-directional bearer) is used (mapped) as a bearerto receive uplink feedback with respect to downlink data (transmissionacknowledgement (ACK or NACK) with respect to transmitted data, orcommunication control information). In the case of uplink communication,similarly, a bi-directional bearer is used (mapped) to receive downlinkfeedback with respect to uplink data. The bearer may be referred to asan AM-DRB in order to clarify the type of the bearer.

As another configuration of the RLC layer, there is a configurationusing an RLC unacknowledged mode (UM). In the RLC UM, there is a case inwhich a bi-directional bearer is used (mapped) similarly to the RLC AM,and there is another case in which a one-directional (uni-directional)bearer is used. In the case of the uni-directional bearer, a single PDCPlayer, a single RLC layer, and a single MAC layer are used (mapped) ineach of downlink communication and uplink communication. The bearer maybe referred to as a UM-DRB in order to clarify the type of the bearer.

As still another configuration of the RLC layer, there is aconfiguration using an RLC transparent mode (TM). This configuration isa configuration in which the RLC layer is not substantially used, anddata in the PDCP layer is directly delivered to the MAC layer in each ofdownlink communication and uplink communication.

For example, an application using transmission control protocol (TCP) toprovide reliable communication is mapped on the RLC AM. For example, anapplication using user data gram protocol (UDP) to provide communicationrequested to have real time characteristics (for example, VoIP) ismapped on the RLC UM.

Further, in the wireless communication system according to theembodiment, the macro base station 100 as the primary wirelesscommunication device separates the data plane between the PDCP layer 101and the RLC layer 102. Specifically, the data plane is separated betweenthe PDCP layer 101 and the RLC layer 102 such that one goes to the RLClayer 102 of the macro base station 100 and the other goes to an RLClayer 201 of the small base station 200.

(Process by Macro Base Station)

A process performed by the communication unit 11 at the time oftransmitting user data to the mobile station 300 will be described.

The communication unit 11 receives pieces of user data of the PDCP layer101 from the higher-layer communication device 4. Then, thecommunication unit 11 regularly assigns, in the PDCP layer 101, numbers(for example, sequence numbers in ascending order) to packets of thereceived pieces of user data. The numbers are also used for a handover.In the embodiment, for example, the communication unit 11 adds oddnumbers in sequence to pieces of data to be sent to the RLC layer 102 ofown device. Further, the communication unit 11 sequentially adds evennumbers to packets of pieces of user data to be sent to the RLC layer201 of the small base station 200. Furthermore, the communication unit11 performs header compression, security check, and encryption on thepieces of user data in the PDCP layer 101.

Then, the communication unit 11 transmits the pieces of user data towhich the odd numbers are added from the PDCP layer 101 to the RLC layer102. Further, the communication unit 11 transmits the pieces of userdata to which the even numbers are added to the RLC layer 201 of thesmall base station 200 via the wired link.

Here, with reference to FIG. 6 , distribution of user data by thecommunication unit 11 will be described. FIG. 6 is a diagram forexplaining the flow of pieces of user data to which the numbers areregularly assigned. Dotted line rectangles in FIG. 6 represent pieces ofdelivered user data. Further, numbers in the pieces of user datarepresent the numbers assigned to the pieces of user data. For example,#1 represents a piece of user data to which the number 1 is assigned.

The communication unit 11 transmits pieces of data to which numbers 1,3, 5, and 7 are assigned in the PDCP layer 101 to the RLC layer 102 asindicated by an arrow 111. Further, the communication unit 11 transmitspieces of data to which numbers 2, 4, 6, and 8 are assigned in the PDCPlayer 101 to the RLC layer 201 of the small base station 200 asindicated by an arrow 112.

Transmission of user data by the communication unit 11 to the RLC layer201 of the small base station 200 will be described in detail below.

The communication unit 11 receives, from the communication unit 31 ofthe mobile station 300, a PDCP status report on information indicating astate of a buffer of the mobile station 300 (for example, a retentionamount of data and a data reception state). Then, the communication unit11 transmits the received PDCP status report to the control unit 14.Thereafter, the communication unit 11 receives, from the control unit14, a notification of an amount of user data to be delivered to the RLClayer 201 of the small base station 200. Then, the communication unit 11transmits the user data with the delivery amount specified by thecontrol unit 14 to the RLC layer 201 of the small base station 200.

Referring back to FIG. 5 , the explanation is continued. Thecommunication unit 11 receives, in the RLC layer 102, pieces of userdata (PDCP PDUs) from the PDCP layer. It is possible to divide orintegrate packets as the pieces of user data to change sizes of thepackets if needed. Further, the communication unit 11 sequentiallyassigns, in the RLC layer 102, numbers for the RLC layer to the packets(PDCP PDUs) received from the PDCP layer. Then, the communication unit11 accumulates the packets (RLC PDUs) as the pieces of user data in abuffer of the RLC layer 102.

Thereafter, the communication unit 11 receives, from the MAC layer 104,a data amount of user data that can be transmitted to the mobile station300. Then, the communication unit 11 transmits pieces of user datacorresponding to the data amount received from the MAC layer 104, fromthe buffer of the RLC layer 102 to the MAC layer 104.

The communication unit 11 assembles, in the MAC layer 104, transmissiondata by using the pieces of user data (RLC PDUs) received from the RLClayer 102 (for example, generates a MAC PDU by adding a MAC header orthe like). Then, the communication unit 11 performs, in the MAC layer104, scheduling for data transmission, and outputs pieces of assembleddata to the mobile station 300 in accordance with the schedule.Incidentally, FIG. 5 illustrates that the MAC layer 104 of thecommunication unit 11 and a MAC layer 301 of the mobile station 300communicate with each other; however, in reality, communication isperformed via a PHY layer or the like.

Next, a process performed by the communication unit 11 at the time ofreceiving user data from the mobile station 300 will be described.

The communication unit 11 performs, in the MAC layer 104, scheduling foruser data reception, and receives pieces of user data from the MAC layer301 of the mobile station 300 in accordance with the scheduling.Subsequently, the communication unit 11 reconstructs (reassembles) thereceived pieces of user data in the MAC layer 104 (for example, removesan MAC header and extracts an RLC PDU from a MAC PDU). Then, thecommunication unit 11 transmits (delivers) the pieces of user data (MACSDUs) from the MAC layer 104 to the RLC layer 103.

The communication unit 11 divides or integrates the received pieces ofuser data (RLC PDUs) in the RLC layer 103. Further, the communicationunit 11 corrects, in the RLC layer 103, the order of the packets byusing the numbers for the RLC layer added to the packets. Then, thecommunication unit 11 transmits (delivers) the pieces of user data (RLCSDUs) in order of the sequence numbers, from the RLC layer 103 to thePDCP layer 101.

The communication unit 11 performs decoding, security check, and headerdecompression on the pieces of user data (PDCP PDUs) in the PDCP layer101.

Then, the communication unit 11 transmits the pieces of user data fromthe PDCP layer 101 to the higher-layer communication device 4.

Further, if a handover occurs, the communication unit 11 releasesmultiple connectivity, and shifts to communication using only singleconnectivity. Then, the communication unit 11 acquires, in the PDCPlayer 101, packets contained in the lower layer, such as the RLC layer,from the lower layer. Then, the communication unit 11 rearranges, in thePDCP layer 101, the acquired packets in numerical order to guarantee thenumbers.

If a handover occurs, a base station operating before move forwards(transfers) packets that have not been transmitted to a base stationthat operates after the move. Then, the base station that operates afterthe move transmits the packet received by the forwarding to the mobilestation 300.

Next, operations of the control unit 14 will be described. The controlunit 14 notifies the mobile station 300 of a parameter setting for thePDCP status report to notify the state of the buffer of the mobilestation 300 (for example, a retention amount of data and a datareception state) via the communication unit 11. Here, the parametersetting for the PDCP status report includes a notification cycle, athreshold for the retention amount to determine whether to send anotification, and the like. In the embodiment, the control unit 14stores, as the threshold for the retention amount, 10% of the buffer ofa PDCP layer 307 of the mobile station 300. Further, in the embodiment,the control unit 14 stores 100 ms as the notification cycle. However,the threshold for the retention amount and the notification cycle may beset to other values, and it is preferable to determine them according tooperations.

Then, after wireless communication with the dual connectivity starts,the control unit 14 receives the PDCP status report of the mobilestation 300 from the communication unit 11. FIG. 7A is a diagramillustrating an example of a PDCP status report for a 12-bit sequencenumber. Further, FIG. 7B is a diagram illustrating an example of a PDCPstatus report for a 15-bit sequence number. FIG. 7C is a diagramillustrating an example of a PDCP status report for a 7-bit sequencenumber. In the PDCP status report, the size of a sequence number isdifferent depending on data to be transmitted and received. For example,in voice over internet protocol (VoIP) or the like, a 7-bit sequencenumber may be used.

In each of PDCP status reports 400, 410, and 420, the size of a firstmissing sequence number (FMS) 401, 411, or 421 is different, whichrepresents a sequence number of the oldest packet among packets thathave not arrived. In the following, the PDCP status report for a 12-bitsequence number illustrated in FIG. 7A will be described as an example.

A format of the PDCP status report 400 illustrated in FIG. 7A is usedsuch that the mobile station notifies the base station of the PDCPsequence numbers. The FMS 401 stores therein a PDCP sequence number ofthe oldest packet among the packets that have not arrived. Incidentally,the PDCP status report 400 corresponds to a 12-bit PDCP sequence number;therefore, the FMS 401 uses 12 bits. Further, Bitmap₁ to Bitmap_(N) areused as options.

In the FMS 401, the sequence number of the oldest packet among thepackets that have not arrived is stored. In other words, the FMS 401stores therein the sequence number of the packet to be received firstamong the packets of the pieces of user data that have not arrived fromthe small base station 200. In the following, the oldest packet amongthe packets that have not arrived may be referred to as an “FMS packet”.Further, in the Bitmap₁ to the Bitmap_(N) of the PDCP status report 400,information indicating arrival states of packets that follow the packetcorresponding to the FMS 401 is stored. In the embodiment, informationin which a bit of “1” indicates arrival and a bit of “0” indicatesnon-arrival is stored in the Bitmap₁ to the Bitmap_(N) of the PDCPstatus report 400. For example, when a certain packet has not arrived,and if arrival and non-arrival are alternately repeated for subsequentsix packets, information of 101010 is stored in the Bitmap₁ to theBitmap_(N).

Further, FIG. 8 is a diagram illustrating information stored in a PDUtype. In a table 430, information on each bit in a PDU type of the PDCPstatus report and contents of the corresponding PDU Type are written. Ifeach bit in the PDU Type is “000”, this indicates a PDCP status reporton the PDCP sequence number of the user data received by the mobilestation 300. Further, if each bit in the PDU Type is “001”, thisindicates an interspersed robust header compression feedback packet(interspersed ROHC feedback packet). The interspersed ROHC feedbackpacket includes feedback information on a PDCP PDU transferred from areception side. Further, each of bits of “010” to “111” in the PDU Typeis retained as a backup. In the embodiment, a bit sequence of “001” isstored in the PDU Type of the PDCP status report 400.

However, it is possible to specify a new different value as a PDU typeto indicate notification of information on the state of the buffer (forexample, a retention amount of data and a data reception state). Forexample, it may be possible to store, in the bit sequence in the PDUType, a value that is determined in advance from among “010” to “111” aseach bit.

Incidentally, in the embodiment, as illustrated in FIG. 7C, the PDCPstatus report 420 for a 7-bit sequence number is generated. Therefore,in the wireless communication system according to the embodiment, evenwhen an RLC unacknowledged mode (UM) bearer is used, it is possible touse the PDCP status report, enabling to improve the reliability toguarantee the order of the packets.

The control unit 14 acquires the number assigned to an FMS packet fromthe FMS 401 of the received PDCP status report 400. Further, the controlunit 14 acquires reception states of packets following the FMS packetfrom the Bitmap₁ to the Bitmap_(N) of the PDCP status report 400.Further, the control unit 14 acquirers a retention amount of data in thebuffer of the mobile station 300 from the amount of packets followingthe packet with the number stored in the FMS 401.

Thereafter, the control unit 14 notifies the communication unit 11 ofthe obtained state of the buffer.

Incidentally, a case will be described, as an example, in which thesequence numbers are assigned to the packets of the pieces of user dataas illustrated in FIG. 6 . For example, it is assumed that a packet witha sequence number of 2 in FIG. 6 has not arrived at the mobile station300.

In this case, packets with the odd sequence numbers are transmitted fromthe macro base station 100, so that the packets with the sequencenumbers 1, 3, 5, and 7 arrive at the mobile station 300. In contrast,packets with the even sequence numbers are transmitted from the smallbase station 200, so that the packets with the sequence numbers 4, 6,and 8 following the number 2 do not arrive at the mobile station 300.This is because if the packet with the number 2 has not arrived at thePDCP layer, the packets with the subsequent numbers are retained in thelower layer and do not arrive at the PDCP layer.

However, in the case of a handover, even if the packet with the number 2has not arrived at the PDCP layer, the packets with the sequence numbers4, 6, and 8 are forwarded to the PDCP layer. Then, in the PDCP layer,the numbers assigned to the packets are guaranteed.

Incidentally, by assuming the worst case, it may be possible to applythe operation at the time of a handover to the case of the dualconnectivity (multiple connectivity). That is, in the case where thesecond packet is not delivered from the macro base station to the smallbase station (for example, a case where a packet loss occurs or a biterror is detected by the small base station side), the packets with thesequence numbers 4, 6, and 8 are not forwarded to the PDCP layer butremain in states like a deadlock. In this case, end-to-endretransmission is performed in a higher-level layer, such as a TCP layeror an application layer.

Therefore, even if a sequence number is lost, by forwarding the packetsto the PDCP layer, it is possible to recognize, in the PDCP layer, whichof the packets has not arrived. For example, in the above-describedexample, if the packet with the sequence number 2 has not arrived at thePDCP layer, the state of the buffer of the PDCP layer indicates “1, 3,4, 5, 6, 7, 8”, and it is found that the packet 2 has not arrived. Ifthe packet 2 has not been delivered to the small base station asdescribed above, the deadlock state occurs as described above.Therefore, to resolve this state, timer control may be employed.

Specifically, for example, if it is detected that the packet with thesequence number 2 has not arrived, a timer is started. Then, if thesecond packet has not arrived within a set time and the timer expires,it is determined that the packet 2 has not arrived at the small basestation, and all of the packets are forwarded to the higher-level layer.

Through the control as described above, it becomes possible to resolvethe deadlock state as soon as possible, and increase the speed ofend-to-end retransmission (to reduce a delay time beforeretransmission).

Incidentally, a setting value of the timer may be set in advance at thetime of setting a line, or may be set at the time of implementing thedual connectivity (multiple connectivity). Further, the timer may bemanaged by the PDCP layer, or by a different layer (the MAC layer or theRLC layer). What is important is to control the timer by the terminal.

In this case, the control unit 14 receives a PDCP status report, inwhich information on the sequence number 2 is stored in the FMS andinformation indicating “101010” is stored in the Bitmap₁ to theBitmap_(N). Then, the control unit 14 acquires the number 2 stored inthe FMS as the number assigned to the FMS packet. Then, the control unit14 confirms, from the bit sequence, that the packets with the sequencenumbers 3, 5, and 7 are accumulated in the buffer of the mobile station300 but the packets with the sequence numbers 4, 6, and 8 have notarrived from the small base station 200. The packets with the sequencenumbers 3, 5, and 7 accumulated in the buffer of the mobile station 300and the packets with the sequence numbers 2, 4, 6, and 8 that have notarrived from the small base station 200 correspond to the retentionamount of data in the buffer of the mobile station 300. Then, thecontrol unit 14 calculates a delivery amount of user data transmittedfrom the macro base station 100 to the small base station 200 by usingthe retention amount of data in the buffer of the mobile station 300.

As for a calculation of the delivery amount of data, for example, thecontrol unit 14 may store therein, in advance, a function such that thedelivery amount is reduced with an increase in the retention amount ofdata and the delivery amount is increased with a decrease in theretention amount of data, and may calculate the delivery amount by usingthe function. Further, the delivery amount for transmission of data maybe fixed, and the control unit 14 may stop delivery of data when theretention amount of data is greater than a predetermined threshold, andthereafter, may resume the delivery of data when the retention amount ofdata falls below the threshold. Further, it may be possible to storecorrespondence between the retention amount of data and the deliveryamount in a stepwise manner, and the control unit 14 may determine thedelivery amount in accordance with the stored correspondencerelationship.

The control unit 14 stores the sequence number assigned to each ofpackets transmitted to the macro base station 100 and the small basestation 200; therefore, it is possible to calculate the retention amountof data on the small base station side on the basis of the received PDCPstatus report, so that it is possible to calculate the delivery amountof data.

(Process by Small Base Station)

Referring back to FIG. 5 , the explanation is continued. A processperformed by the small base station 200 at the time of transmitting userdata to the mobile station 300 will be described. In FIG. 5 , only theRLC layer 201, an RLC layer 202, and a MAC layer 203 used for theexplanation are illustrated among the layers of the small base station200. Incidentally, the RLC layer 201 is a downlink RLC layer, and theRLC layer 202 is an uplink RLC layer (the RLC layer associated withdownlink).

The communication unit 21 receives, in the RLC layer 201, pieces of userdata transmitted from the PDCP layer 101 of the macro base station 100via the wired link.

Subsequently, the communication unit 21 receives, in the RLC layer 201,pieces of user data (PDCP PDUs) from the PDCP layer. The communicationunit 21 may divide or integrate packets as the pieces of user data tochange sizes of the packets if needed. Further, the communication unit21 sequentially assigns, in the RLC layer 201, numbers for the RLC layerto the packets (PDCP PDUs) received from the PDCP layer. Then, thecommunication unit 21 accumulates the packets (RLC PDUs) as the piecesof user data in the buffer of the RLC layer 201.

Thereafter, the communication unit 21 receives, from the MAC layer 203,a data amount of user data that can be transmitted to the mobile station300. Then, the communication unit 21 transmits pieces of user datacorresponding to the data amount received from the MAC layer 203, fromthe buffer of the RLC layer 201 to the MAC layer 203.

The communication unit 21 assembles, in the MAC layer 203, transmissiondata by using the pieces of user data (RLC PDUs) received from the RLClayer 201 (for example, generates a MAC PDU by adding a MAC header orthe like). Then, the communication unit 21 performs, in the MAC layer203, scheduling for data transmission, and outputs pieces of assembleddata (RLC SDUs) to the mobile station 300 in accordance with theschedule.

Next, a process performed by the small base station 200 at the time ofreceiving user data from the mobile station 300 will be described.

The communication unit 21 performs, in the MAC layer 203, scheduling foruser data reception, and receives pieces of user data from a MAC layer302 of the mobile station 300 in accordance with the scheduling.Subsequently, the communication unit 21 reconstructs (reassembles) thereceived pieces of user data in the MAC layer 203. Then, thecommunication unit 21 transmits (delivers) the pieces of user data (MACSDUs) from the MAC layer 203 to the RLC layer 202.

The communication unit 21 divides or integrates the received pieces ofuser data (RLC PDUs) in the RLC layer 202. Further, the communicationunit 21 corrects, in the RLC layer 202, the order of the packets byusing the numbers for the RLC layer added to the packets. Then, thecommunication unit 21 transmits (delivers) the pieces of user data (RLCSDUs) in order of the sequence numbers, from the RLC layer 202 to thePDCP layer 101 of the macro base station 100 via the wired link.

(Process by Mobile Station)

Next, the mobile station 300 will be described. In FIG. 5 , the MAClayer 301, the MAC layer 302, RLC layers 303 to 306, and the PDCP layer307 used for the explanation are illustrated among the layers of themobile station 300. Incidentally, in the embodiment, the mobile station300 has a function to receive pieces of data from two base stations inparallel. The MAC layer 301, the RLC layer 303, the RLC layer 304, andthe PDCP layer 307 are layers for transmitting and receiving data to andfrom the macro base station 100. The RLC layer 303 is a downlink RLClayer, and the RLC layer 304 is an uplink RLC layer. Further, the MAClayer 302, the RLC layer 305, the RLC layer 306, and the PDCP layer 307are layers for transmitting and receiving data to and from the smallbase station 200. The RLC layer 305 is a downlink RLC layer, and the RLClayer 306 is an uplink RLC layer (the RLC layer associated withdownlink).

A process performed by the mobile station 300 at the time of receivinguser data from the macro base station 100 will be described.

The communication unit 31 performs, in the MAC layer 301, scheduling foruser data reception, and receives pieces of user data from the MAC layer104 of the macro base station 100 in accordance with the scheduling.Subsequently, the communication unit 31 reconstructs (reassembles) thereceived pieces of user data in the MAC layer 301. Then, thecommunication unit 31 transmits the pieces of user data (MAC SDUs) fromthe MAC layer 301 to the RLC layer 303.

The communication unit 31 divides or integrates the received pieces ofuser data (RLC PDUs) in the RLC layer 303. Further, the communicationunit 31 corrects, in the RLC layer 303, the order of the packets byusing the numbers for the RLC layer added to the packets. Then, thecommunication unit 31 transmits (delivers) the pieces of user data (RLCSDUs) in order of the sequence numbers, from the RLC layer 303 to thePDCP layer 307.

The communication unit 31 performs, in the PDCP layer 307, decoding,security check, and header decompression on the pieces of user data.

Then, the communication unit 31 performs data processing, such asdisplay of the pieces of data or calculations using the pieces of data,on the received pieces of user data.

Next, a process performed by the mobile station 300 at the time oftransmitting user data to the macro base station 100 will be described.

The communication unit 31 regularly assigns, in the PDCP layer 307,numbers (for example, sequence numbers in ascending order) to packets ofpieces of user data to be transmitted. Further, the communication unit31 performs header compression, security check, and encryption on thepieces of user data in the PDCP layer 307.

Then, the communication unit 31 transmits the pieces of user data fromthe PDCP layer 307 to the RLC layer 304.

Subsequently, the communication unit 31 receives, in the RLC layer 304,pieces of user data (PDCP PDUs) from the PDCP layer. The communicationunit 31 may divide or integrate packets as the pieces of user data tochange sizes of the packets if needed. Further, the communication unit31 sequentially assigns, in the RLC layer 304, numbers for the RLC layerto the packets (PDCP PDUs) received from the PDCP layer. Then, thecommunication unit 31 accumulates the packets (RLC PDUs) as the piecesof user data in a buffer of the RLC layer 304.

Thereafter, the communication unit 31 receives, from the MAC layer 301,a data amount of user data that can be transmitted to the macro basestation 100. Then, the communication unit 31 transmits pieces of userdata corresponding to the data amount received from the MAC layer 301,from the buffer of the RLC layer 304 to the MAC layer 301.

The communication unit 31 assembles, in the MAC layer 301, transmissiondata by using the pieces of user data (RLC PDUs) received from the RLClayer 304 (for example, generates a MAC PDU by adding a MAC header orthe like). Then, the communication unit 31 performs, in the MAC layer301, scheduling for data transmission, and outputs pieces of assembleddata to the macro base station 100 in accordance with the schedule.

A process performed by the mobile station 300 at the time of receivinguser data from the small base station 200 will be described.

The communication unit 31 performs, in the MAC layer 302, scheduling foruser data reception, and receives pieces of user data from the MAC layer203 of the small base station 200 in accordance with the scheduling.Subsequently, the communication unit 31 reconstructs (reassembles) thereceived user data in the MAC layer 302. Then, the communication unit 31transmits (delivers) the pieces of user data (MAC SDUs) from the MAClayer 302 to the RLC layer 305.

The communication unit 31 divides or integrates the received pieces ofuser data (RLC PDUs) in the RLC layer 305. Further, the communicationunit 31 corrects, in the RLC layer 305, the order of the packets byusing the numbers for the RLC layer added to the packets. Then, thecommunication unit 31 transmits (delivers) the pieces of user data (RLCSDUs) in order of the sequence numbers, from the RLC layer 305 to thePDCP layer 307.

The communication unit 31 performs, in the PDCP layer 307, decoding,security check, and header decompression on the pieces of user data(PDCP PDUs).

Then, the communication unit 31 performs data processing, such asdisplay of the pieces of data or calculations using the pieces of data,on the received pieces of user data.

Next, a process performed by the mobile station 300 at the time oftransmitting user data to the small base station 200 will be described.

The communication unit 31 regularly assigns, in the PDCP layer 307,numbers (for example, sequence numbers in ascending order) to packets ofpieces of user data to be transmitted. Further, the communication unit31 performs header compression, security check, and encryption on thepieces of user data in the PDCP layer 307.

Then, the communication unit 31 transmits the pieces of user data fromthe PDCP layer 307 to the RLC layer 306.

Subsequently, the communication unit 31 receives, in the RLC layer 306,pieces of user data (PDCP PDUs) from the PDCP layer. The communicationunit 31 may divide or integrate packets as the pieces of user data tochange sizes of the packets if needed. Further, the communication unit31 sequentially assigns, in the RLC layer 306, numbers for the RLC layerto the packets (PDCP PDUs) received from the PDCP layer. Then, thecommunication unit 31 accumulates the packets (RLC PDUs) as the piecesof user data in a buffer of the RLC layer 306.

Thereafter, the communication unit 31 receives, from the MAC layer 302,a data amount of user data that can be transmitted to the macro basestation 100. Then, the communication unit 31 transmits pieces of userdata corresponding to the data amount received from the MAC layer 302,from the buffer of the RLC layer 306 to the MAC layer 302.

The communication unit 31 assembles, in the MAC layer 302, transmissiondata by using the pieces of user data (RLC PDUs) received from the RLClayer 306 (for example, generates a MAC PDU by adding a MAC header orthe like). Then, the communication unit 31 performs, in the MAC layer302, scheduling for data transmission, and outputs pieces of assembleddata (RLC SDUs) to the small base station 200 in accordance with theschedule.

Next, notification of information indicating a data state (for example,a retention amount of data and a data reception state) to the macro basestation 100 by the mobile station 300 will be described.

The control unit 34 receives a parameter setting for the PDCP statusreport via the communication unit 31. Then, the control unit 34 setsvarious parameters, such as a notification cycle for periodicnotification and a threshold for the retention amount, for the PDCPstatus report by using the parameter setting for the PDCP status report.

The control unit 34 monitors the retention amount in the buffer in thePDCP layer 307 with respect to the packets received in the PDCP layer307, at the time of receiving user data. Then, the control unit 34, whenthe retention amount exceeds the threshold for the retention amount,acquires a number assigned to the FMS packet among packets of pieces ofuser data that have not arrived from the small base station 200.Further, the control unit 34 acquires reception states of packets to beprocessed subsequent to the FMS packet.

Then, the control unit 34 generates the PDCP status report 400 in theformat as illustrated in FIG. 7A, by using the acquired information.

Here, an example will be described in which the control unit 34generates the PDCP status report when sequence numbers are assigned topieces of user data as illustrated in in FIG. 6 and the packet with thesequence number 2 has not arrived at the mobile station 300.

In this case, the packets with the sequence numbers 1, 3, 5, and 7arrive at the mobile station 300, but the packets with the sequencenumbers 4, 6, and 8 do not arrive at the mobile station 300. This isbecause if the packet with the number 2 has not arrived at the PDCPlayer, the packets with the subsequent numbers are retained in the lowerlayer and do not arrive at the PDCP layer.

However, in the case of a handover, even if the packet with the number 2has not arrived at the PDCP layer, the packets with the sequence numbers4, 6, and 8 are absorbed by the PDCP layer. Then, in the PDCP layer, thenumbers assigned to the packets are guaranteed.

Therefore, the control unit 34 acquires the number 2 as the numberassigned to the FMS packet among the packets of the pieces of user datathat have not arrived from the small base station 200. Further, thecontrol unit 34 acquires, as the reception states of the packets to beprocessed subsequent to the FMS packet, information indicating that thepackets with the sequence numbers 1, 3, 5, and 7 have been received butthe packets with the sequence numbers 4, 6, and 8 have not beenreceived. Incidentally, the control unit 34 may acquire the numberassigned to the FMS packet among the pieces of user data that have notarrived from the macro base station 100, and acquire the receptionstates of the packets to be processed subsequent to the FMS packet.

Then, the control unit 34 stores, in an area of the FMS in the PDCPstatus report, information indicating the packet with the sequencenumber 2. Further, the control unit 34 stores 101010 in each of the bitsin the Bitmap₁ to the Bitmap_(N) from the top. Further, the control unit34 stores, in the area of the PDU Type, data that indicates a packet fornotifying information on the retention amount in the buffer.

Thereafter, the control unit 34 transmits the generated PDCP statusreport to the communication unit 31.

The communication unit 31 transmits the PDCP status report received fromthe control unit 34 to the macro base station 100.

Further, the control unit 34 generates the PDCP status report in eachcycle as stored. Then, the communication unit 31 transmits the PDCPstatus report to the macro base station 100.

Incidentally, in the embodiment, as a timing to notify the retentionamount in the buffer, two timings, that is, a time when a threshold isexceeded and a periodic time, are used; however, the notification timingis not limited to this example. For example, the control unit 34 mayemploy only the periodic time as the notification timing. Further, thecontrol unit 34 may employ the time when the threshold is exceeded asthe notification timing, and send notifications at predeterminedintervals until the threshold is not exceeded.

Next, with reference to FIG. 9 , the overall flow to control thedelivery amount of data at the time of dual connectivity will bedescribed. FIG. 9 is a sequence diagram for explaining the overall flowof the delivery amount of data. Herein, a case will be described inwhich the macro base station 100 stops transmission of user data to thesmall base station 200 when the retention amount of data in the bufferof the mobile station 300 exceeds a threshold, and thereafter resume thetransmission of user data when the buffer of the mobile station 300becomes empty. Here, states 501 to 503 in FIG. 9 represent dataretention states in the buffer of the mobile station 300, and states 511to 514 represent data retention states in the buffer of the macro basestation 100. Further, arrows and pieces of data with respect to thestates 511 to 514 indicate that pieces of user data are transmitted fromthe buffer of the macro base station 100 to the small base station 200at this time.

The macro base station 100 transmits user data to the small base station200 via the wired link (Step S1). The small base station 200 transmitsthe user data to the mobile station 300 (Step S2).

At this time, data is accumulated in the buffer of the macro basestation 100 as in the state 511. Then, the user data is transmitted fromthe buffer of the macro base station 100. Further, in the buffer of themobile station 300, a small amount of data relative to the capacity ofthe buffer is accumulated as in the state 501.

Furthermore, the macro base station 100 transmits user data to the smallbase station 200 via the wired link (Step S3). The small base station200 transmits the user data to the mobile station 300 (Step S4). In thiscase, however, a delay occurs in the transmission of data from the smallbase station 200 to the mobile station 300.

Therefore, in this case, the amount of data accumulated in the buffer ofthe mobile station 300 increases as in the state 502. In this state, theuser data is accumulated in the buffer of the macro base station 100 asin the state 512, and user data is transmitted from the buffer of themacro base station 100.

Then, if the amount of data accumulated in the buffer of the mobilestation 300 exceeds a threshold, the mobile station 300 transmits, tothe macro base station 100, information indicating the retention amountof data in the buffer (Step S5). In response to this, the macro basestation 100 stops transmission of user data to the small base station200.

Therefore, while data is accumulated in the buffer of the macro basestation 100 as in the state 513, user data is not transmitted from thebuffer of the macro base station 100. Then, the mobile station 300receives delayed data from the small base station 200 and performs dataprocessing, so that the retention amount of data in the buffer isreduced. Then, as in the state 503, no data is retained in the buffer ofthe mobile station 300.

Thereafter, the mobile station 300 transmits, to the macro base station100, information indicating that the retention amount of data in thebuffer is zero, at a periodic timing to notify the retention amount ofdata (Step S6).

The macro base station 100, upon confirming that the retention amount ofdata in the buffer of the mobile station 300 is zero, resumes thetransmission of user data to the small base station 200 (Step S7). Atthis time, data is accumulated in the buffer of the macro base station100 as in the state 514, and the accumulated user data is transmittedfrom the buffer. Then, the small base station 200 transmits the userdata to the mobile station 300 (Step S8).

Next, with reference to FIG. 10 , control on an amount of user datadelivered from the macro base station 100 to the small base station 200that are connected by dual connectivity in the communication systemaccording to the embodiment will be explained. FIG. 10 is a flowchart tocontrol the amount of user data delivered to the small base station inthe communication system according to the second embodiment.

The control unit 14 of the macro base station 100 instructs thecommunication unit 11 to manage the amount of user data delivered to thesmall base station 200 by regularly assigning sequence numbers (StepS101).

The transmitting unit 13 regularly assigns, in the PDCP layer 101, thesequence numbers to packets of pieces of user data, and transmits thepieces of user data to the small base station 200 via the wired link(Step S102). The small base station 200 transmits the pieces of userdata received from the macro base station 100 to the mobile station 300.

The transmitting unit 13 transmits a parameter setting including athreshold, a notification cycle, and the like for a PDCP status reportto the mobile station 300 (Step S103).

The receiving unit 32 of the mobile station 300 receives the parametersetting for the PDCP status report from the macro base station 100 (StepS104).

The control unit 34 acquires the parameter setting for the PDCP statusreport from the receiving unit 32. Then, the control unit 34 setsparameters, such as the threshold and the notification cycle, for thePDCP status report (Step S105).

The control unit 34 determines whether the retention amount of data inthe buffer of the mobile station 300 exceeds the threshold or whetherthe notification cycle has come (Step S106). If the retention amount ofdata does not exceed the threshold and if the notification cycle has notcome (NO at Step S106), the control unit 34 waits until the retentionamount of data exceeds the threshold or the notification cycle comes.

In contrast, if the retention amount of data exceeds the threshold or ifthe notification cycle has come (YES at Step S106), the control unit 34generates a PDCP status report for notifying information indicating theretention amount of data. Then, the transmitting unit 33 transmits thePDCP status report generated by the control unit 34 to the macro basestation 100 (Step S107).

The receiving unit 12 of the macro base station 100 receives the PDCPstatus report for notifying the information indicating the retentionamount of data from the mobile station 300 (Step S108).

The control unit 14 acquires the PDCP status report from the receivingunit 12. Then, the control unit 14 calculates the retention amount ofdata in the buffer of the mobile station 300 by using the PDCP statusreport (Step S109).

Subsequently, the control unit 14 determines, from the calculatedretention amount of data, an amount of user data delivered to the smallbase station 200 (Step S110).

The transmitting unit 13 receives a notice of the amount of user datadelivered to the small base station 200 determined by the control unit14. Then, the transmitting unit 13 changes the amount of user datadelivered to the small base station 200 to a specified delivery amount(Step S111). Then, the transmitting unit 13 transmits, to the small basestation 200, pieces of user data assigned with the regular sequencenumbers, in accordance with the changed delivery amount.

Incidentally, in the flowchart in FIG. 10 , the flow of a singlechanging process is described to explain the process of changing thedelivery amount; however, in reality, the macro base station 100 and themobile station 300 repeat the processes from Step S106 to Step S111 inFIG. 10 .

Next, with reference to FIG. 11A and FIG. 11B, a comparison betweentransmission of user data from the macro base station 100 to the smallbase station 200 with the dual connectivity in the communication systemaccording to the embodiment and conventional dual connectivity will bedescribed. FIG. 11A is a diagram for explaining transmission of userdata at the time of dual connectivity in a conventional communicationsystem. Further, FIG. 11B is a diagram for explaining transmission ofuser data at the time of dual connectivity in the communication systemaccording to the second embodiment.

A connection between a macro base station 110 and a small base station210 in FIG. 11A and a connection between the macro base station 100 andthe small base station 200 in FIG. 11B are connections indicated bydashed arrows, and are established by using wired links.

As illustrated in FIG. 11A, in the conventional communication system,the MAC layer of the small base station 210 acquires a data size that isdetermined by the PHY layer from a wireless quality (which is calleduplink channel information (UCI) and corresponds to channel qualityinformation (CQI) as a downlink wireless quality or a sounding referencesignal (SRB) as an uplink wireless quality, which is measured by amobile station). Then, the RLC layer of the small base station 210acquires, from the MAC layer, the data size for transmission, andtransmits user data corresponding to the data size to the MAC layer.

Then, the size of the user data transmitted from the RLC layer to theMAC layer of the small base station 210 is notified to the macro basestation 110 via the wired link. The PDCP layer of the macro base station110 transmits user data to the RLC layer of the small base station 210in accordance with the size of the user data sent from the RLC layer ofthe small base station 210.

In this case, the macro base station 110 determines a delivery amount ofuser data to be transmitted to the RLC layer of the small base station210, by using the information transmitted from the small base station210 via the wired link. In the wired link, transmission of informationmay be delayed depending on the quality of communication. That is, theinformation transmitted from the small base station 210 via the wiredlink may be delayed, and if the delay occurs, it is difficult for themacro base station to appropriately transmit user data according to thetransmission state of the small base station 210.

In contrast, as illustrated in FIG. 11B, in the communication systemaccording to the embodiment, the MAC layer of the small base station 200acquires a data size that is determined by the PHY layer from a wirelessquality. Then, the RLC layer of the small base station 200 acquires,from the MAC layer, the data size for transmission, and transmits userdata corresponding to the data size to the MAC layer.

Further, information indicating a retention amount of data in the bufferin the PDCP layer of the mobile station 300 is wirelessly transmittedfrom the mobile station 300 to the macro base station 100. Then, themacro base station 100 determines the delivery amount of user data fromthe information on the retention amount of data in the buffer in thePDCP layer of the mobile station 300, and transmits the user data withthe delivery amount to the RLC layer of the small base station 200.

In this case, the macro base station 100 determines the delivery amountof user data to be transmitted to the the RLC layer of small basestation 200, by using the information wirelessly transmitted from themobile station 300. The radio speed is higher than the speed of a wiredlink that has a low communication quality. That is, in the communicationsystem according to the embodiment, the macro base station 100 canacquire the information for determining the delivery amount of user datato be transmitted to the the RLC layer of small base station 200 at ahigher speed as compared to the conventional technology. Therefore, themacro base station 100 according to the embodiment can appropriatelytransmit user data according to the transmission state of the small basestation 200.

As described above, the wireless communication system according to theembodiment determines, in the dual connectivity, the delivery amount ofuser data to be transmitted from the primary wireless communicationstation to the secondary wireless communication station by using theretention amount of data in the buffer of the mobile station. Therefore,the primary wireless communication station can transmit user data withan appropriate delivery amount from the primary wireless communicationstation to the secondary wireless communication station. Therefore,according to the wireless communication system of the embodiment, it ispossible to improve the communication efficiency between the wirelesscommunication stations.

Further, the retention amount of data in the buffer is notified by usinga PDCP status report as a known signal (signaling); therefore, it ispossible to realize various functions as described above by only addinga small change to a known system. That is, the wireless communicationsystem according to the embodiment can be constructed easily.

(Hardware Configuration)

FIG. 12 is a hardware configuration diagram of a base station. Examplesof the base station include the wireless communication devices 1 and 2illustrated in FIG. 1 , and the macro base station 100 and the smallbase station 200 illustrated in FIG. 4 .

The base station includes an antenna 901, a control unit 902, an RFcircuit 903, a memory 904, a CPU 905, and a network interface 906.

The control unit 902 realizes the functions of the control unit 14illustrated in FIG. 1 and FIG. 4 , for example.

The network interface 906 is an interface for connecting a network by awired link. For example, the macro base station 100 and the small basestation 200 are connected by the wired link via the network interface906.

The CPU 905, the memory 904, and the RF circuit 903 realize thefunctions of the communication unit 11 including the receiving unit 12and the transmitting unit 13, and the functions of the communicationunit 21 including the receiving unit 22 and the transmitting unit 23illustrated in FIG. 1 and FIG. 4 .

For example, the memory 904 stores therein various programs, such as aprogram for realizing the functions of the communication unit 11 or thecommunication unit 21.

The CPU 905 reads the program stored in the memory 904, and realizes thefunctions of the communication unit 11 or the communication unit 21 incooperation with the RF circuit 903 or the like.

FIG. 13 is a hardware configuration diagram of a mobile station.Examples of the mobile station include the wireless communication device3 illustrated in FIG. 1 and the mobile station 300 illustrated in FIG. 4.

The mobile station includes an antenna 911, a control unit 912, an RFcircuit 913, a memory 914, and a CPU 915.

The control unit 912 realizes the functions of the control unit 34illustrated in FIG. 1 and FIG. 4 , for example.

The CPU 915, the memory 914, and the RF circuit 913 realize thefunctions of the communication unit 31 including the receiving unit 32and the transmitting unit 33 illustrated in FIG. 1 and FIG. 4 .

For example, the memory 914 stores therein various programs, such as aprogram for realizing the functions of the communication unit 31.

The CPU 915 reads the program stored in the memory 914, and realizes thefunctions of the communication unit 31 in cooperation with the RFcircuit 913 or the like.

(Modification)

In the above-described second embodiment, a case has been described inwhich the macro base station 100 serves as the primary base station andthe small base station 200 serves as the secondary base station.However, the configurations of the base stations are not limited to thisexample. For example, even in a system configuration as illustrated inFIG. 14 , the wireless communication system according to the embodimentcan operate.

FIG. 14 is a schematic diagram illustrating dual connectivity in awireless communication system according to a modification of the secondembodiment. The wireless communication system according to themodification includes a small base station 200A as the wirelesscommunication device 1 in FIG. 1 . Further, a small base station 200B isprovided as the wireless communication device 2 in FIG. 1 . Furthermore,the mobile station 300 is provided as the wireless communication device3 in FIG. 1 .

The mobile station 300 connects to the small base station 200A as aprimary base station. The mobile station 300 is connected to the smallbase station 200A by the control plane 352 indicated by a solid arrowand the user plane 351 indicated by a dashed arrow. Further, the mobilestation 300 connects to the small base station 200B as a secondary basestation. The mobile station 300 is connected to the small base station200B by the user plane 351.

The configuration of the wireless communication system as illustrated inFIG. 14 is usually employed to improve the characteristics of the uplinkcommunication.

Further, while the small base station 200 is connected to the macro basestation 100 in FIG. 14 , the small base station 200 may be directlyconnected to a higher-layer communication device of the macro basestation 100.

Third Embodiment

Next, a third embodiment will be described. A wireless communicationsystem according to the third embodiment differs from the wirelesscommunication system of the second embodiment in that a data plane isseparated between the RLC layer and the MAC layer. In the descriptionbelow, explanations of the components with the same functions will beomitted.

FIG. 15 is a diagram illustrating transmission and reception of userdata performed by using each of link layers in the wirelesscommunication system according to the third embodiment.

The communication unit 11 of the macro base station 100, in the case ofdownlink communication, transmits packets of pieces of user data, towhich numbers are regularly assigned in the PDCP layer 101, to the RLClayer 102.

Then, the communication unit 11 receives a delivery amount of user datafrom the control unit 14. Subsequently, the communication unit 11performs a process of dividing or integrating packets or adding numbersin the RLC layer. Thereafter, the communication unit 11 extracts thepackets to which the numbers are added in the PDCP layer 101 as packetsto be transmitted to the small base station 200. Then, the communicationunit 11 transmits, to the MAC layer 203 of the small base station 200,the extracted packets of pieces of user data with the delivery amountthat is specified by the control unit 14 via the wired link. Further,the communication unit 11 outputs, to the MAC layer 104, packets towhich numbers are assigned so as to be processed in the own station.

In the case of uplink communication, the communication unit 11 receives,in the RLC layer 103, pieces of user data from the MAC layer 203 of thesmall base station 200 via the wired link. Then, the communication unit11 divides or integrates packets of the received pieces of user data andarranges them in numerical order. Thereafter, the communication unit 11transmits the pieces of user data received from the small base station200, from the RLC layer 103 to the PDCP layer 101.

The communication unit 21 of the small base station 200, in the case ofdownlink communication, acquires, in the MAC layer 203, pieces of userdata from the RLC layer 102 of the macro base station 100 via the wiredlink. Then, the communication unit 21 transmits the pieces of user datafrom the MAC layer 203 to the mobile station 300.

In the case of uplink communication, the communication unit 21 of thesmall base station 200 transmits the pieces of user data received fromthe mobile station 300, from the MAC layer 203 to the RLC layer 103 ofthe macro base station 100.

In this manner, even when the data plane is separated between the RLClayer and the MAC layer, it is possible to determine the amount of datadelivered to the secondary wireless communication station according tothe retention amount of data in the buffer of the mobile station. Thatis, the primary wireless communication station can transmit pieces ofuser data with the appropriate delivery amount from the primary wirelesscommunication station to the secondary wireless communication station.Therefore, the wireless communication system according to the embodimentcan improve the communication efficiency between wireless communicationstations.

Fourth Embodiment

Next, a fourth embodiment will be described. A wireless communicationsystem according to the fourth embodiment differs from the wirelesscommunication system of the second embodiment in that a data plane isseparated in a stage before the PDCP layer. In the description below,explanations of the components with the same functions will be omitted.

FIG. 16 is a diagram illustrating transmission and reception of userdata performed by using each of link layers in the wirelesscommunication system according to the fourth embodiment.

In the case of downlink communication, the communication unit 11 of themacro base station 100 receives a delivery amount of user data from thecontrol unit 14. Then, the communication unit 11 transmits, to a PDCPlayer 204 of the small base station 200, packets of pieces of user datareceived from the higher-layer communication device 4 with the deliveryamount that is specified by the control unit 14. At this time, thecommunication unit 11 distributes the pieces of user data such that thenumbers added in the PDCP layers 101 and 204 are guaranteed. Forexample, if it is determined that even numbers are added in own stationand odd numbers are added in the small base station 200, thecommunication unit 11 transmits, to the small base station 200, thepackets that are assigned with the odd numbers when the packets arearranged.

Further, the communication unit 11 transmits the rest of the pieces ofuser data to the PDCP layer 101 of the own station.

Subsequently, the communication unit 11 adds numbers to packets of thepieces of user data according to a rule defined in the PDCP layer 101.For example, the communication unit 11 adds even numbers to the packets.Then, the communication unit 11 transmits the packets of the pieces ofuser data from the PDCP layer 101 to the RLC layer 102.

In the case of uplink communication, the communication unit 11 receivespieces of user data from the PDCP layer 204 of the small base station200 via the wired link. Then, the communication unit 11 transmits thereceived pieces of user data to the higher-layer communication device 4.

The communication unit 21 of the small base station 200, in the case ofdownlink communication, acquires, in the PDCP layer 204, pieces of userdata from the macro base station 100 via the wired link. Then, thecommunication unit 21 adds numbers to packets of the pieces of user dataaccording to a rule defined in the PDCP layer 204. For example, thecommunication unit 21 adds odd numbers to the packets in the PDCP layer204. Then, the communication unit 21 transmits the pieces of user datafrom the PDCP layer 204 to the MAC layer 203.

In the case of uplink communication, the communication unit 21 of thesmall base station 200 transmits the pieces of user data from the RLClayer 202 to the PDCP layer 204.

Then, the communication unit 21 performs decoding or security check onthe pieces of user data in the PDCP layer 204. Then, the communicationunit 21 transmits the pieces of user data from the PDCP layer 204 to themacro base station 100.

In this manner, even when the data plane is separated in a stage beforethe PDCP layer, it is possible to determine the amount of data deliveredto the secondary wireless communication station according to theretention amount of data in the buffer of the mobile station. That is,the primary wireless communication station can transmit pieces of userdata with the appropriate delivery amount from the primary wirelesscommunication station to the secondary wireless communication station.Therefore, the wireless communication system according to the embodimentcan improve the communication efficiency between wireless communicationstations.

Fifth Embodiment

Next, a wireless communication system according to a fifth embodimentwill be explained. The wireless communication system according to theembodiment differs from the second embodiment in that a retention amountof data in an RLC buffer of the small base station is notified by usingRLC feedback information.

FIG. 17 is a diagram illustrating transmission and reception of userdata performed by using each of link layers in the wirelesscommunication system according to the fifth embodiment. In theembodiment, as illustrated in FIG. 17 , an example will be described inwhich a data plane is separated between the RLC layer and the MAC layerin the macro base station 100. In the following, the same functions asthose of the second embodiment will be omitted.

The communication unit 11 of the macro base station 100 separates a dataplane after the RLC layer 102, and transmits pieces of user data to thesmall base station 200. At this time, the communication unit 11transmits the pieces of user data with a delivery amount specified bythe control unit 14.

The control unit 14, at the start of dual connectivity, notifies themobile station 300 of a setting of an RLC status report that is the RLCfeedback information via the communication unit 11.

Thereafter, the control unit 14 manages the number of packetstransmitted during the dual connectivity by using a counter, and setsflags to packets to be transmitted to the mobile station 300 when thecounter reaches a certain value. FIG. 18 is a diagram illustrating anexample of a format of a transmission packet. For example, the controlunit 14 sets a flag of P (Poll) 701 of a transmission packet 700 in FIG.18 to “1”. The P 701 is a bit for setting a polling command. The controlunit 14 transmits the packet to which the flag is set to the mobilestation 300, to notify the mobile station 300 of transmission of an RLCstatus report. In normal data, for example, the flag of the P 701 is setto “0”.

Further, the control unit 14 calculates the amount of transmitted data,and if the amount of transmitted data exceeds a predetermined value,sets a flag in the same manner as in the case where the counter reachesa certain value. In this case, the control unit 14 clears the amount oftransmitted data after setting the flag, and repeatedly calculates theamount of subsequently-transmitted data.

Alternatively, a timing to request transmission of an RLC status reportis not specifically limited. For example, the control unit 14 may set aflag at a constant period.

The control unit 14 acquires the RLC status report from thecommunication unit 11. Then, the control unit 14 acquires, from the RLCstatus report, the sequence number of the oldest packet that has notbeen received by the mobile station 300. Further, if a packet loss hasoccurred, the control unit 14 acquires a sequence number of a lostpacket among divided packets.

Incidentally, the RLC status report for transmitting informationindicating the retention amount of data in the buffer of the mobilestation 300 will be explained. FIG. 19 is a diagram illustrating aformat of the RLC status report. An RLC status report 600 includes anACK_SN 601. The ACK_SN 601 indicates a sequence number of a nextnon-received RLC data PDU. However, this indicates a sequence number ofan RLC PDU (RLC data PDU) for which non-reception has not been notifiedin a STATUS PDU. That is, once “non-reception” is reported by the STATUSPDU, this sequence number is not set as the ACK_SN 601.

Further, the RLC status report 600 includes a NACK_SN 602. The NACK_SN602 indicates a sequence number of an AMD PDU (or a part thereof) forwhich a loss is detected by an AM RLC entity on the reception side.

Further, SOstart indicates a part of an AMD PDU with a sequence numberfor which a loss is detected by an AM RLC entity on the reception side(NACK_SN; for example, NACK_SN related to SOstart). Further, SOendindicates a part of an AMD PDU with a sequence number for which a lossis detected by an AM RLC entity on the reception side (NACK_SN; forexample, NACK_SN related to SOend).

Furthermore, the RLC status report 600 includes a carrier packet type(CPT).

FIG. 20 is a diagram illustrating contents corresponding to a valuestored in the CPT. As illustrated in FIG. 20 , if the value of the CPTis “001”, it indicates feedback information on a secondary connection.Further, if the value of the CPT is “002”, it indicates feedbackinformation on a third connection. Furthermore, values of the CPT from“003” to “111” are reserves. In the embodiment, a bit sequence of “001”is stored in the CPT of the RLC status report 600.

However, it may be possible to specify a new different value as a CPT toindicate notification of information on the retention amount of data.For example, it may be possible to determine, in advance, any of “003”to “111” of the CPT as a value on the retention amount of data in thebuffer of the mobile station 300, and store the predetermined value inthe CPT when the information on the retention amount of data is to benotified.

The control unit 14 checks whether the value of the CPT of the acquiredRLC status report 600 is a value that is determined in advance and thatindicates an RLC status report for transmitting the information on theretention amount of data in the buffer of the mobile station 300. Then,the control unit 14 checks the ACK_SN 601 and acquires a sequence numberof the oldest packet that has not arrived at the mobile station 300.Thereafter, the control unit 14 specifies packets that follow thesequence number of the oldest packet that has not arrived at the mobilestation 300, and obtains a retention amount of data in the RLC buffer ofthe small base station 200. Further, for example, the control unit 14obtains a total amount of areas indicated by NACK_SN, SOstart, and SOendthrough conversion to bytes, and obtains the retention amount of data inthe RLC buffer of the small base station 200.

FIG. 21 is a diagram for explaining an example of a method ofcalculating a retention amount of data. In FIG. 21 , hatched portionsindicate pieces of data whose delivery is not confirmed. If atransmission side receives the RLC status report, the followingcalculations are possible.

Specifically, a retention amount of an RLC PDU=(b₁+h₃)+(b₄+h₂) or(SOend+h₃)+(b₄+h₂). Further, a retention amount of an RLC SDU=B₁+B₂.Furthermore, a retention amount of a PDCP PDU=B₁+B₂. Moreover, aretention amount of a PDCP SDU=(B₁−H₁)+(B₂−H₂).

Then, the control unit 14 determines a delivery amount of user datadepending on the retention amount of data in the buffer of the smallbase station 200. Then, the control unit 14 notifies the communicationunit 11 of the determined delivery amount. Incidentally, among theabove-described retention amounts, any of the retention amounts may beused to determine the delivery amount of user data.

The control unit 34 of the mobile station 300 receives a setting of anRLC status report from the macro base station 100 at the start of thedual connectivity. Then, the control unit 34 sets notification of theRLC status report.

Then, the control unit 34 receives, from the macro base station 100, apacket to which a flag for requesting transmission of the RLC statusreport is set, while the dual connectivity is implemented. Then, thecontrol unit 34 generates the RLC status report by using a sequencenumber of the received packet and a sequence number of a packet that isdetected as having been lost. Thereafter, the control unit 34 performsRLC feedback by transmitting the generated RLC status report to thesmall base station 200 via the communication unit 31, and transmits theRLC status report to the macro base station 100.

Next, with reference to FIG. 22 , the overall flow to transmit the RLCstatus report will be described. FIG. 22 is a sequence diagram forexplaining the overall flow to transmit the RLC status report in thewireless communication device according to the fifth embodiment. Here,states 801 and 802 in FIG. 22 represent data retention states in the RLCbuffer of the small base station 200. Further, arrows and pieces of datawith respect to the states 801 and 802 indicate that pieces of user dataare transmitted from the RLC buffer of the small base station 200 atthis time.

The macro base station 100 transmits, to the mobile station 300, apacket with a flag set to “0”, in order to notify that transmission ofthe RLC status report is not requested (Step S11). Further, the macrobase station 100 transmits user data to the small base station 200 (StepS12). The small base station 200 transmits the user data to the mobilestation 300 (Step S13).

At this time, data is accumulated in the RLC buffer of the small basestation 200 as in the state 801. Then, the user data is output from theRLC buffer of the small base station 200.

Thereafter, if the number of packets exceeds a certain value or theamount of transmitted data exceeds a threshold, the macro base station100 transmits, to the mobile station 300, a packet with a flag set to“1”, in order to notify a request for transmission of the RLC statusreport (Step S14). Thereafter, the macro base station 100 continues totransmit user data to the small base station 200 (Step S15). The smallbase station 200 continues to transmit user data to the mobile station300 (Step S16).

Even at this time, data is accumulated in the RLC buffer of the smallbase station 200 as in the state 802. Then, the user data is output fromthe RLC buffer of the small base station 200.

Then, upon receiving the packet with the flag of “1”, the mobile station300 transmits the RLC status report to the macro base station 100 (StepS17). Further, the mobile station 300 performs RLC feedback bytransmitting the RLC status report to the small base station 200 (StepS18).

Thereafter, the macro base station 100 obtains a retention amount ofdata in the RLC buffer of the small base station 200 from the receivedRLC status report, and controls an amount of data delivered to the smallbase station 200 according to the obtained retention amount of data.

As described above, the primary wireless communication station accordingto the embodiment determines an amount of user data delivered to thesecondary wireless communication station, according to the retentionamount of data in the RLC buffer of the secondary_wireless communicationstation by using the RLC status report. Therefore, the primary wirelesscommunication station can transmit user data with an appropriatedelivery amount from the primary wireless communication station to thesecondary wireless communication station. Consequently, the wirelesscommunication system according to the embodiment can improve thecommunication efficiency between wireless communication stations.

(Modification)

In the fifth embodiment, the macro base station 100 transmits a packet,to which a flag is set, to request transmission of the RLC statusreport; however, it is not limited thereto. For example, the small basestation 200 may transmit a packet, to which a flag is set, to requesttransmission of the RLC status report.

In this case, the control unit 24 of the small base station 200 maymonitor the number of packets or the amount of transmitted data todetermine whether to set a flag, similarly to the control unit 14 of themacro base station 100 of the second embodiment.

Alternatively, similarly to the fifth embodiment, the macro base station100 may determine whether to set a flag and transmit a packet, to whichthe flag is set, to the small base station 200, and the small basestation 200 may transfer the packet to the mobile station 300.

Moreover, while the mobile station 300 wirelessly transmits the RLCstatus report to the macro base station 100 in the fifth embodiment, thetransmission method is not limited to this example. For example, it maybe possible to transmit the RLC status report to the macro base station100 by using the wired link.

Furthermore, while a polling command conventionally defined as a packetformat is used to request transmission of the RLC status report in thefifth embodiment, the method to request transmission is not limited tothis example. For example, it may be possible to define a new pollingcommand to request transmission.

Moreover, it may be possible to cause the mobile station 300 to performPDCP status reporting instead of a status report at the RLC level, tocause the macro base station 100 to obtain the retention amount of datain the RLC buffer of the secondary wireless communication station.

The embodiments disclosed above may be arbitrarily combined withoutdeparting from the scope and spirit of the invention. Further, downlinkcommunication is mainly explained and uplink communication associatedwith the downlink communication is also explained; however, when uplinkcommunication is performed and downlink communication is associated withthe uplink communication, the embodiments can be applied in the samemanner.

According to an embodiment of a wireless communication system, a mobilestation, a base station, and a wireless communication method of thedisclosed technology, it is possible to improve communication efficiencybetween wireless communication stations.

All examples and conditional language provided herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventors to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

What is claimed is:
 1. A base station configured to communicate datawith a mobile station, comprising: a controller; a communicatorconfigured to transmit data to the mobile station; a transmitterconfigured to transfer a portion of data to be transmitted to the mobilestation to another wireless communication apparatus; and a receiverconfigured to receive, from the mobile station, a PDCP status report oninformation indicating a state of a buffer of the mobile station fromthe mobile station, wherein the receiver configured to transmit thereceived PDCP status report to the controller, and the controllerconfigured to notify the mobile station of a parameter setting for thePDCP status report, and the parameter setting for the PDCP status reportincludes a parameter to determine whether to send the PDCP statusreport, the PDCP status report includes a first missing sequence numberrepresenting a sequence number of the oldest packet among packets thathave not arrived for the other wireless communication apparatus, and asize of the first missing sequence number has a plurality of sequencelengths.
 2. The base station according to claim 1, wherein the PDCPstatus report further includes information indicating an arrival stateof packets following the packet corresponding to the sequence number. 3.The base station according to claim 1, the controller further configuredto: confirm packets accumulated in the buffer of the mobile station andthe packets that have not arrived from the other wireless communicationapparatus; and calculate a delivery amount of user data to betransmitted from the base station to the other wireless communicationapparatus by using the retention amount of data in the buffer of themobile station.
 4. The base station according to claim 3, wherein thecontroller further configured to: stop transmission of user data to theother wireless communication apparatus in response to informationindicating the retention amount of data in the buffer of the mobilestation, the information being transmitted from the mobile station, whenthe retention amount of data accumulated in the buffer of the mobilestation exceeds the threshold; and resume the transmission of user datato the other wireless communication apparatus upon confirming that theretention amount of data in the buffer of the mobile station is zero. 5.The base station according to claim 1, wherein one of the sequencelength of the first missing sequence number is either of 7 bit length,12 bit length or 15 bit length.
 6. A mobile station comprising: acommunicator configured to perform multiple communications with a firstwireless communication apparatus and a second wireless communicationapparatus (200) to which data is transferred from the first wirelesscommunication apparatus; and a transmitter configured to transmit, tothe first wireless communication apparatus, a PDCP status report oninformation indicating a state of a buffer of the mobile station,wherein the first wireless communication apparatus notifies the mobilestation of a parameter setting for the PDCP status report, wherein theparameter setting for the PDCP status report includes a parameter todetermine whether to send the PDCP status report, the PDCP status reportincludes a first missing sequence number representing a sequence numberof the oldest packet among packets that have not arrived for the anotherwireless communication apparatus and a size of the first missingsequence number has a plurality of sequence lengths.
 7. The mobilestation according to claim 6, wherein the PDCP status report furtherincludes information indicating an arrival state of packets followingthe packet corresponding to the sequence number.
 8. The mobile stationaccording to claim 6, wherein one of the sequence length of the firstmissing sequence number is either of 7 bit length, 12 bit length or 15bit length.